1. Field of the Invention
The present invention relates to a magnification varying optical system used for optical units and the like, and to an image taking apparatus having this magnification varying optical system.
2. Description of the Related Art
In recent years, with the spread of personal computers (PCs), digital still cameras (image taking apparatuses) with which images can be easily taken have been widespread. Such digital still cameras (DSCs) are also required to be smaller in size and thickness and to be higher in performance, for example, to have a high zooming function or a high aberration correcting function like cameras using silver halide film. For this reason, lens optical systems mounted on digital still cameras such as magnification varying optical systems are also required to be smaller in size and higher in performance.
It can be said that magnification varying optical systems in which a plurality of lens units are arranged in a line, so-called straight optical systems like those of Patent Documents 1 and 2 are unfit to be mounted on DSCs. This is because when the direction of the overall length of the magnification varying optical system and the direction of the depth of the DSC coincide with each other, the DSC becomes thick.
Therefore, recently, to realize the reduction of the thickness of DSCS, a magnification varying optical system has been developed the overall length of which is reduced by bending the light ray traveling from the object side to the image side, by an optical prism. Such a magnification varying optical system is disclosed, for example, in Patent Document 3. Such a magnification varying optical system the overall length of which is suppressed can be appropriately disposed in the limited space in the housing of the DSC. Consequently, the size of the housing and consequently, the size of the DSC are smaller and thinner.                [Patent Document 1] Japanese Laid-Open Patent Application No. 2004-246043        [Patent Document 2] Japanese Laid-Open Patent Application No. 2005-156828        [Patent Document 3] U.S. Pat. No. 7,079,328 B2        
The optical power-arrangement of the magnification varying optical system of Patent Document 3 is negative, positive, negative from the object side to the image side, and realizes focusing at a short distance by moving the third lens unit of negative optical power from the object side, toward the image side. For this reason, in this magnification varying optical system, the shortest object distance is easily reduced.
However, in such a magnification varying optical system, a prism having a concave surface on the object side is situated on the most object side in the most object side first lens unit. For this reason, off-axial aberration due to this concave surface is apt to be caused. Then, to correct such off-axial aberration, it is necessary to provide many aspherical surfaces in the magnification varying optical system or to increase the overall length of the magnification varying optical system. For this reason, it cannot be said that the overall length of the magnification varying optical system of Patent Document 3 is sufficiently reduced. That is, it cannot be said that the magnification varying optical system of Patent Document 3 sufficiently satisfies the requirements, size reduction and performance enhancement.